Set Aside от Guido Berhörster

Set Aside can be used to quickly set aside open tabs from the current window and restore them later.

Tabs which were set aside can be managed via the sidebar which shows the collections of tabs in the order of creation, including the number of tabs belonging to the collection, its creation date and time, as well as thumbnail previews of each tab.

Note that tabs which are set aside are closed, this may lead to a loss of data if e.g. there is unsubmitted form data. Tabs with privileged protocols such as file:, moz-extension:, or about: cannot be set aside and will be left open because the extension would not be able to open them again due to browser security policy.